ObjectivesUsing routinely collected hospital data, this study explored secular trends over time in breast feeding initiation in a large Australian sample. The association between obesity and not breast feeding was investigated utilising a generalised estimating equations logistic regression that adjusted for sociodemographics, antenatal, intrapartum and postpartum conditions, mode of delivery and infant’s-related covariates.DesignPopulation-based retrospective panel.SettingA regional hospital that serves 26% of Victoria’s 6.5 million population in Australia.ParticipantsAll women experiencing live births between 2010 and 2017 were included. Women with missing body mass index (BMI) were excluded.ResultsA total of 7491 women contributed to 10 234 live births. At baseline, 57.2% of the women were overweight or obese, with obesity increasing over 8 years by 12.8%, p=0.001. Although, breast feeding increased over time, observed in all socioeconomic status (SES) and BMI categories, the lowest proportions were consistently found among the obese and morbidly obese (78.9% vs 87.1% in non-obese mothers, p<0.001). In the multivariable analysis, risk of not breast feeding was associated with higher BMI, teenage motherhood, smoking, belonging to the lowest SES class, gravidity >4 and undergoing an assisted vaginal or caesarean delivery. Compared with women with a normal weight, the obese and morbidly obese were 66% (OR 1.66, 95% CI 1.40 to 1.96, p<0.001) to 2.6 times (OR 2.61, 95% CI 2.07 to 3.29, p<0.001) less likely to breast feed, respectively. The detected dose–response effect between higher BMI and lower breast feeding was not explained by any of the study covariates.ConclusionThis study provides evidence of increasing breast feeding proportions in regional Victoria over the past decade. However, these proportions were lowest among the obese and morbidly obese and those coming from the most disadvantaged backgrounds suggesting the need for targeted interventions to support breast feeding among these groups. The psychosocial and physiological associations between obesity and breast feeding should further be investigated.